The Renazzo meteorite

Amercan Museum

ovitates

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 7QTH STREET, NEW YORK 24, N.Y.

NUMBER 2106 OCTOBER 10, 1962

The Renazzo Meteorite
By Brian MAson! anp H. B. Wik?
INTRODUCTION

The Renazzo meteorite fell near Ferrara, Italy, on January 15, 1824.
A number of stones fell, the largest weighing about 5 kilograms. It was
described by Cordier (1827), with an analysis by Laugier, and there are
numerous references to it in the nineteenth-century literature, which is
enumerated by Wilfing (1897). Wiilfing was able to account for some
1083 grams in collections, the largest amount (441 grams) being in the
Mineralogical Institute of the University of Bologna. The fate of the
major portion of this meteorite remains unknown, which is unfortunate,
because the Renazzo meteorite is unique in several respects, as can be
seen from the following description.

MINERALOGICAL COMPOSITION

In hand specimen the Renazzo meteorite has a striking appearance,
consisting of comparatively large white chondrules, up to 3 mm. in
diameter, in a black structureless groundmass. In thin section (fig. 1)
the chondrules are seen to consist for the most part of granular olivine.
Occasional chondrules are made up of clinoenstatite (showing well-
developed polysynthetic twinning) and enstatite, and others consist of a

1 Chairman, Department of Mineralogy, the American Museum of Natural History.
2 Research Associate, Department of Mineralogy, the American Museum of Natural
History.

2 AMERICAN MUSEUM NOVITATES NO. 2106

Fic. 1. Thin section of the Renazzo meteorite, showing chondrules of olivine
and pyroxene in opaque groundmass of carbonaceous serpentine. Photograph
by G. R. Adlington. x 6.

mixture of grains of clinoenstatite, enstatite, and olivine. The chondrules
are frequently bordered by a rim of granules of nickel-iron (fig. 2). One
olivine chondrule is completely mantled by a border of nickel-iron about
0.3 mm. thick (fig. 3). Another chondrule has an iron core 0.5 mm. in
diameter and a rim, | mm. thick, of granular olivine (fig. 4). The ground-
mass is black and opaque; an X-ray powder photograph of this material

1962 MASON AND WIIK: METEORITE 3

shows lines of serpentine and magnetite. Notes on the individual minerals
follow.

OuivinE: The refractive indices are: a = 1.638, y = 1.670, indi-
cating that the olivine is almost iron-free and close to Mg,SiO, (forsterite)
in composition. This was confirmed by the X-ray technique of Yoder
and Sahama (1957); the diffraction pattern showed sharp peaks, indicating
olivine of uniform composition, and the positions of the peaks corresponded
to 96 mol per cent Mg,SiQ,.

CLINOENSTATITE: This mineral is prominent in some of the chondrules
by its low birefringence and the presence of polysynthetic twinning; the
individual twin lamellae are very thin. The refractive indices could not
be precisely determined because of the closely spaced twinning lamellae,
but are close to those of pure MgSiQO, .

EnstaTITE: This mineral is difficult to identify in thin section, but was
readily detected in the acid-insoluble fraction of the meteorite, along with
the clinoenstatite. It is untwinned, with extinction parallel to the cleavage
traces. The refractive indices are: a9 = 1.653, y = 1.661, indicating that
it is nearly pure MgSiO,, with little or no iron content.

DiopsipE: Crushed fragments of the meteorite show rare grains with
higher indices than the olivine, enstatite, or clinoenstatite, oblique
extinction, and birefringence about 0.03. The refractive indices are:
a = 1.674, y = 1.704, and the mineral is almost certainly diopside.

SERPENTINE: As mentioned above, this mineral has been identified by
X-ray powder photographs as a principal phase in the opaque ground-
mass. Its identification as serpentine rather than chlorite rests on (a)
the absence of a 14A reflection; and (b) the low Al,O, content of the
meteorite, as shown by the analysis (table 1). From the analysis it appears
that the serpentine must have a considerable iron content.

NickEL-IRon: This is present almost or entirely as kamacite. It com-
monly occurs as rims of small granules around the individual chondrules,
and the interior of such chondrules is usually free from nickel-iron.
Other chondrules do not have nickel-iron rims, but then show numerous
small kamacite particles throughout (these may be chondrules that
have been cut tangentially). Sometimes the nickel-iron forms large
round grains—in effect, metal chondrules. Nickel-iron is notably absent
from the groundmass between the chondrules.

TRoiLitE: This mineral is present in lesser amount than is usual in
chondrites. It occurs principally as small grains in association with
nickel-iron, and as dust-like particles in the groundmass between the
chondrules.

PENTLANDITE: This mineral occurs in small amount in association with

4 AMERICAN MUSEUM NOVITATES NO. 2106

Fic. 2. Polished surface of the Renazzo meteorite; the chondrules are promi-
nent, and are frequently mantled by grains (white) of nickel-iron. Photograph
by Battelle Memorial Institute. « 7.

1962 MASON AND WIIK: METEORITE 5

%.

>
at re.

Fic. 3. A chondrule in the Renazzo meteorite, consisting of a core of olivine
(white), 0.6 mm. in diameter, mantled by a rim of nickel-iron (gray), 0.3 mm.
thick. Photograph by J. Weber.

troilite. Its presence is surprising, in view of the considerable excess of
free iron; under these circumstances the nickel would normally alloy
with the metal, rather than forming a sulphide.

Macnetite: This mineral is evidently present in considerable amount
in the opaque groundmass, to judge from the X-ray powder photographs.
The cell dimension is a = 8.44 A, considerably higher than that of pure
Fe,O, and close to that of trevorite, NiFe,O, (a = 8.43 A), which suggests
that some of the nickel in the meteorite is present in the magnetite phase.

CHROMITE: Occurs in small amount, in idiomorphic crystals.

GRAPHITE: This mineral appears to be abundant in the groundmass,
in minute, almost submicroscopic grains.

A notable feature of the Renazzo meteorite is the apparent absence of

6 AMERICAN MUSEUM NOVITATES NO. 2106

TABLE 1
CHEMICAL COMPOSITION OF THE RENAZZO METEORITE

A B Cc

Fe 10.72 H 0.63 Mg 33.94
Ni 1.35 Cc 1.44 Si 32.32
Co 0.114 N 0.06 Fe 25.71
Cu 0.0145 Na 0.41 Al 2.66
FeS 3.59 Mg 14.33 Ca 1.85
SiO, 33.83 Al 1.25 Ni 1.32
TiO, 0. 186 Si 15.805 Na 1.02
AlO, 2.36 P 0.122 Cr 0.43
FeO 15.35 S 1.31 P 0.23
MnO 0.24 K 0.034 Mn 0.20
MgO —-23.76 Ca 1.272 Ti 0.13
CaO 1.78 Ti 0.111 Co 0.10
Na,O 0.55 V 0.027 K 0.05
K,O 0.042 Cr 0.383 V 0.03
P.O, 0.28 Mn 0.186 Cu 0.01
H,O 5.67 Fe 24.93
Cr,0, 0.56 Co 0.114 100.00
V,0; 0.048 Ni 1.35
C 1.44 Cu 0.0145
N 0.06 (O 36.22)

101.94 100.00

A Chemical analysis expressed as nickel-iron, troilite, and oxides

B Chemical analysis expressed as elements, with oxygen added to make 100 per cent

C Chemical analysis expressed as atom percentages with the elimination of H, O, C,
and S

plagioclase. This mineral, which is almost universally present in chondritic
meteorites in amounts of from 5 to 10 per cent, could not be detected
either in thin sections or in the acid-insoluble residues.

The density of the meteorite was determined by measuring the apparent
loss of weight on suspension in carbon tetrachloride and found to be 3.29.

CHEMICAL COMPOSITION

The chemical analysis is given in table 1, in the conventional form
expressed as oxides, troilite, and metal; in terms of the individual ele-
ments as determined by analysis, with oxygen to bring the total to 100;
and recalculated as atom percentages with the elimination of H, O, S,
and C. The conventional form of presenting meteorite analyses involves

1962 MASON AND WIIK: METEORITE 7

Fic. 4. A chondrule in the Renazzo meteorite, consisting of a core of nickel-iron
(black), 0.5 mm. in diameter, mantled by granular olivine (white). Photograph
by J. Weber.

certain assumptions, for example, that all S is present as FeS, that Fe
in excess of free metal and FeS is present as FeO, and that the H,O
given by the analysis is present as free or combined H,O. These assump-
tions are probably valid for most chondritic meteorites, but are certainly
not for Renazzo. Much of the S is present as FeS, but some is in pentland-
ite, and some may be present as complex organic compounds. Some Fe
is certainly present as ferric iron in magnetite, and the serpentine phase

8 AMERICAN MUSEUM NOVITATES NO. 2106

may well contain ferric iron. The H,O given by the analysis is in part
combined in serpentine, but some of it is probably present in combination
with carbon as organic compounds (the high summation of the conven-
tional form of the analysis can be ascribed to hydrogen combined with
carbon rather than oxygen). Under these circumstances the form of
presentation in column B in table 1 is preferable, since it gives the results
actually obtained by the analysis. In effect, the chemical analysis deter-
mines the amounts of the different elements, except the amount of oxygen,
no readily applicable method for this element being available.

The expression of the analysis as atomic percentages after eliminating
H, O, S, and C was used by one of us (Wiik, 1956) for comparing analyses
of the different types of chondrites. Such a procedure in effect distin-
guishes non-volatile elements from those likely to be lost or gained during
heating in extra-terrestrial environments. The figures for the Renazzo
meteorite show that its elemental composition is closely similar to that
of Murray, a Type II carbonaceous chondrite.

DISCUSSION

The well-developed chondrules in the Renazzo meteorite show that it
belongs beyond doubt in the group of the chondritic meteorites. However,
it is not readily placed in any of the five classes of chondrites commonly
recognized—enstatite, olivine-bronzite, olivine-hypersthene, _ olivine-
pigeonite, or carbonaceous chondrites (Mason, 1962). In mineralogy and
elemental composition it resembles the Type II carbonaceous chondrites
of Wiik (1956), except that these contain little or no free metal, and the
chondrules are normally small and sparse. The abundant free metal and
the iron-free pyroxenes and olivine in Renazzo are properties analogous
to those of the enstatite chondrites. In fact, the Renazzo meteorite appears
to be intermediate between a carbonaceous chondrite and an enstatite
chondrite.

The nature of the Renazzo meteorite poses some significant questions.
Some of its remarkable features are: (a) the occurrence of chondrules of
essentially iron-free anhydrous magnesium silicates in an iron-rich
groundmass of hydrated silicate; (b) the mantling of many chondrules
by a sheath of nickel-iron granules, whereas the interior of the chondrules
are free from nickel-iron; (c) the constant association of nickel-iron with
the chondrules and its absence from the groundmass.

Several hypotheses can be formulated to explain the genesis of the
Renazzo meteorite. The most likely possibilities seem to be:

1. The chondrules were formed in one environment, the groundmass

1962 MASON AND WIIK: METEORITE 9

in a different environment, and the meteorite is a chance accumulation
of these two materials with different genetic histories.

2. The meteorite is the product of a single process, the chondrules and
the groundmass being genetically related, in which case there are the
following alternatives: (a) the groundmass has been formed from the
materials of the chondrules; (b) the chondrules have been formed from
the material of the groundmass.

The first possibility is consistent with the theory of Urey (1959),
according to which the chondritic meteorites are agglomerates of
debris resulting from collisions between pre-existing bodies. However,
there are weighty arguments against this in the case of Renazzo. Its
elemental composition as expressed in C in table | is similar to that of
other carbonaceous chondrites, and to the H-group olivine-pyroxene
chondrites studied by Urey and Craig (1953). Chance accumulation of
debris from different objects would hardly result in such uniformity of
chemical composition. An additional argument against Renazzo’s
representing a chance accumulation is the uniformity of composition of
the chondrules; they consist of MgSiO, or Mg,SiO, or mixtures of these.
Such chondrules are rare in meteorites, being found only in some of the
carbonaceous chondrites and in the small group of enstatite chondrites.
On the evidence the first possibility must be rejected as an explanation
for the origin of the Renazzo meteorite.

The second possibility now requires consideration: that chondrules
and groundmass are genetically related, which raises the question as to
whether the chondrules formed from the groundmass, or vice versa. One
of us (Mason, 1960a, 1960b) has argued as a general postulate that
chondrules are the product of a solid-state recrystallization of carbo-
naceous hydrated silicates, similar in composition to the groundmass of
Renazzo and the carbonaceous chondrites. These views have been strongly
opposed by Urey (1961), who suggests . . . “‘that some water-carrying
organic compounds, hydrogen sulphide, etc., infiltrated some high-iron-
group chondritic material, oxidized the metallic iron to magnetite or the
sulphide, deposited carbon compounds, sulphate, sulphur, etc., and
removed some sodium and potassium, and partly destroyed the chon-
drules.”’

The Renazzo meteorite may well be a critical object in a decision
between these two contrasted hypotheses. The evidence from this meteorite
seems to favor the hypothesis that the chondrules formed from the ground-
mass. The following formula shows a perfectly feasible chemical reaction:

10 AMERICAN MUSEUM NOVITATES NO. 2106

Mg,Si,O,,(OH), + Fe,O, + 2C =
2Mg,SiO, + 2MgSiO, + 3Fe + 4H,O + 2CO,

serpentine + magnetite + carbon = olivine + pyroxene + iron

groundmass chondrules

However, the reaction is presumably reversible, proceeding to the left
at low temperatures and to the right at high temperatures. Bowen and
Tuttle (1949), who studied the phase relations in the system MgO-SiO,-
H,O, showed that serpentine can decompose to give Mg,SiO, at tempera-
tures as low as 400° C.

In view of the presumed reversibility of the chemical reaction it is not
possible to decide on this basis alone whether the chondrules formed
from the groundmass or vice versa. However, the structural relations
between the minerals in the chondrules and the groundmass indicate that
the chondrules have formed from the groundmass, and not that the
groundmass is the product of partial replacement of the chondrules.
Several features support this view. Most of the chondrules are above
average diameter for chondritic meteorites. Chondrules average about
1 mm. in diameter in most meteorites, whereas those in Renazzo are
frequently larger, which is inconsistent with the belief that they were
being destroyed with the formation of groundmass serpentine. The con-
centration of metallic nickel-iron marginally to the chondrules indicates
that the chondrules were probably in the process of growth rather than
of destruction, the crystallization of the pure magnesium silicates as
chondrules concurrently with the formation of the metal resulting in the
concentration of the latter on the margins of the chondrules. The distri-
bution and form of the nickel-iron suggest that it has formed from the
groundmass, not that it is in the process of being converted into magnetite.
It is noteworthy that no free metal is visible in the groundmass itself.
One might expect to see occasional vestigial remains were the iron a
primary phase from which the groundmass was formed. Other features
of the mineralogy are inconsistent with a derivation from high-iron-group
chondritic material. Such material never contains chondrules of iron-free
olivine, which are the commonest type in Renazzo; such material always
contains plagioclase feldspar or its glassy equivalent, and this is absent
from Renazzo.

Considering all the evidence, we conclude that the Renazzo meteorite
represents an arrested stage in the conversion of a carbonaceous chondrite
into an enstatite chondrite. The chondrules of iron-free olivine and
pyroxene with the associated nickel-iron appear to have formed at the

1962 MASON AND WIIK: METEORITE 1]

expense of the carbonaceous serpentine groundmass, which is presumably
the result of a thermal metamorphism that failed to proceed to completion.

ACKNOWLEDGMENTS

Professor P. Gallitelli of the University of Bologna kindly supplied the
material for this research. We wish to thank Professor Paul Ramdohr for
examining a polished surface of this meteorite and for providing us with
much information on the nature of the opaque minerals. Our thanks are
also due to Mr. G. R. Adlington, Mr. J. Weber, and the Battelle Memorial
Institute for microphotographs. We are indebted to the National Science
Foundation for a grant (NSF-G14547) toward the expenses of this
investigation.

REFERENCES

Bowen, N. L., AND O. F. TuTTLe
1949. The system MgQO-SiO,-H,O. Bull. Geol. Soc. Amer., vol. 60, pp.
439-460.
Corptrr, L.
1827. Rapport fait 4 PAcadémie des Sciences, sur une pierre meteorique
tombée prés de Ferrare en 1824. Ann. Chim. et Phys., vol. 34, pp.

132-139.
Mason, B.
1960a. Origin of chondrules and chondritic meteorites. Nature, vol. 186,
pp. 230-231.

1960b. The origin of meteorites. Jour. Geophys. Res., vol. 65, pp. 2965-2970.
1962. The classification of chondritic meteorites. Amer. Mus. Novitates,
no. 2085, pp. 1-20.

Urey, H. C.
1959. Primary and secondary objects. Jour. Geophys. Res., vol. 64, pp.
1721-1737.

1961. Criticism of Dr. B. Mason’s paper on “The origin of meteorites.”
Ibid., vol. 66, pp. 1988-1991.
Urey, H. C., anp H. Craic
1953. The composition of the stone meteorites and the origin of the meteorites.
Geochim. et Cosmochim. Acta, vol. 4, pp. 36-82.
Wurk, H. B.
1956. The chemical composition of some stony meteorites. Geochim. et
Cosmochim. Acta, vol. 9, pp. 279-289.
Wi trina, E. A.
1897. Die Meteoriten in Sammlungen und ihre Literatur. Tubingen,
Laupp’schen Buchhandlung, 461 pp.
Yooper, H. S8., anp T. G. SAHAMA
1957. Olivine X-ray determinative curve. Amer. Min., vol. 42, pp. 475-491.